1. Field of the Invention
The invention relates to electric circuit fusing devices, especially for high voltage, high current applications.
2. Description of the Prior Art
The term "fuse" implies a protective device for interrupting the flow of an electric current when, due to overload or circuit faults, a current flow in a protected circuit exceeds a certain design value.
Four different types of high-power, high-voltage circuit fuses are presently common in industry and power utility uses. These may be categorized as follows:
1. Oil expulsion fuses PA1 2. Air expulsion fuses PA1 3. Energy limiter fuses PA1 4. Vacuum fuses
Oil expulsion fuses involve placement of the expendable fuse element in a tank of oil. While these have certain advantages in respect to containment of the by-products of fuse operation and containment of accompanying sounds, they also have major disadvantages. Among these disadvantages is the very limited interrupting capability. For fault currents above the level of approximately 3000 amperes, this type of fuse can fail to clear, continuing to arc under oil. The resulting sustained fault current can cause circuit damage, and arcing within the oil container with consequent buildup pressure therein.
In overhead systems such as primary and secondary overhead distribution systems, the air expulsion fuse is often employed. These devices produce relatively high sound levels and expel substantial amounts of by-product during operation. It is necessary that the "blow" of such a device produce a substantial gap to prevent sustained arcing which would prevent the fuse from clearing. Such a device does not lend itself to compact mounting and the operational sound levels are also undesirable in many instances.
Energy limiter fuses have certain characteristics which make them very attractive for the so-called "dead-front" switch application. There are difficulties however, in achieving flexibility and loadability. For example, there are only a limited number of full range clearing fuse sizes available at the present time. Also, the low amperage ratings tend to limit full utilization of the 200 ampere capability of the taps.
The vacuum fuse is a relatively new device and at present it is constructed similar to a vacuum interrupter with two stationary contacts joined by a copper connecting link all enclosed in a vacuum bottle. There is no means provided for mechanically separating the stationary contacts as would be the case in a vacuum interrupter, the operation of the device depending upon the melting of the copper link. Initially at least, an arc is established when the link melts, but once the energy available for the system is less than the energy need to sustain the lengthening arc, the vacuum fuse will effectively clear. The high dielectric strength and dielectric recovery rate in the vacuum fuse prevents restrike or reignition. Typical current ratings are on the order of 150 amperes, 200 amperes, 250 amperes and possibly as much as 300 amperes. Interrupt capability is on the order of 12 KA symmetrical. Typical voltage ratings are 8.3 KV and 15 KV. Vacuum fuses have certain advantages over the other prior art fuses aforementioned, and have very good coordination with transformer explosion fuses.
In all of the foregoing prior art fusing devices, the device is either totally expendable or requires reprocessing to be readied for additional service.
The manner in which the present invention offers great improvement over the prior art devices will be understood as this description proceeds.